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Aerobic and anaerobic bacteria can be identified by growing them in liquid culture:
1. Obligate aerobe
2. Obligate anaerobe
3. Facultative anaerobic organism (continuum with "Facultative aerobic organism")
4. Microaerophile
5. Aerotolerant

A facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation. In contrast, obligate anaerobes die in presence of oxygen.

Some examples of facultative anaerobic bacteria are Staphylococcus (Gram positive), Escherichia coli (Gram negative), Corynebacterium (Gram positive), and Listeria (Gram positive). Certain eukaryote phyla are also facultative anaerobes, including fungi such as yeasts and many aquatic invertebrates such as Nereid (worm) polychaetes, for example.There are also circulating white blood cells that are classified as facultative anaerobes. These include neutrophils, monocytes and tissue macrophages.

The concentrations of oxygen and fermentable material in the environment influence the organism's use of aerobic respiration vs. fermentation to derive energy. In brewer's yeast, the Pasteur shift is the observed cessation of oxygen consumption when fermentable sugar is supplied. In a growing culture, the energy "economics" disfavors respiration due to the "overhead cost" of producing the apparatus, as long as sufficient fermentable substrate is available, even though the energy output per mole of fermented material is far less than from respiration's complete oxidation of the same substrate.

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